A major concern with the utilization of certain fuels to directly fire conventional power generation systems is the particulates produced by combustion of the fuels. These particulates remain in the combustion gas stream. Because the gas stream running such systems can adversely impact on the life of turbines and the like, the gas stream should be substantially free of particulate matter. Although conventional devices such as cyclones may be used to remove some of the larger solid particulate matter from combustion gas streams, these devices generally fail to remove the smaller particulates from the streams. Similar problems also exist in many gas streams in which the particulate suspended matter originates from other than combustion.
Removal of solid particulate from a gas stream is most important in coal-fuel advanced power generation systems. Particularly, a direct coal-fired gas turbine which uses gas turbines coupled in series with advanced coal combustors has the potential to achieve high thermodynamic cycle efficiencies. The nature of the coal-based fuels, however, has prevented efficient operation and effectiveness of these direct coal-fired gas turbine systems. Conventional gas turbine systems normally employ clean, premium-grade petroleum distillates in the combustion system. In contrast, coal-based fuels produce ash and chemical species such as sulfur and fuel-bound nitrogen not found in appreciable quantities in the petroleum-based fuels. This mineral matter in coal-based fuels potentially impairs gas turbine efficiency, reduces reliability, increases maintenance costs, and adversely impacts the environment. Degradation of the coal-fired gas turbine's airfoil efficiency also occurs through corrosion, deposition, and erosion brought about by particulates and other materials in the gas stream.
Direct coal firing of combustion gas turbines requires means to reduce or eliminate erosion of the turbine blades due to the presence of fly ash and other particulates in the gas stream created by the burning of coal. If such erosion is not reduced, the turbine blade's life span becomes very short, on the order of 100 hours, thus compromising the economic viability of directly coal firing combustion gas turbines.
Direct coal firing may also result in the release of alkali vapors and sulfur compounds in addition to particulate combustion products. Such contaminant emissions cause the turbine blades to corrode quickly.
Fuel-bound nitrogen causes nitrogen oxide (NO.sub.x) emissions to also form in the gas stream. Although nitrogen oxides do not affect turbine blades per se, they do represent pollutants that are not desirable in the atmosphere. Methods and processes to either reduce the production of nitrogen oxides or to destroy or remove such pollutants from the flue gas stream are necessary to meet the requirements of the Clean Air Act. Economically viable means for removing pollutants from the turbine exhaust before discharging such exhaust into the atmosphere have not heretofore been available.
Various attempts have been made to overcome the above and other problems to provide an economically feasible and efficient process for direct solid fuel firing of gas turbines. Attempts have also been made to provide a method for removing fine particulates from a gas stream. For example, coal fuels have been ultracleaned prior to combustion to reduce coal-based contaminants. This, of course, imposes substantial financial burdens as well as delays in time for utilization of the coal. In one such approach, the coal is extensively cleaned in an attempt to remove ash and sulfur from the fuel prior to firing. A cold water slurry is made from micronized, deeply cleaned coal and then used as fuel. This approach, of course, is expensive but does produce an essentially oil-like slurry fuel made from coal that requires little modification to the gas turbine engine, The cost of the necessary coal cleaning and slurry preparation, however, is sufficiently high that this approach has essentially been abandoned.
Other attempts to create clean combustion gas streams have utilized modestly clean fuel products in connection with a hot gas clean-up system upstream from the gas turbine. Most of the particulate control devices are secondary or tertiary particulate control devices in that multiple clean-up stages are required to sufficiently clean the particulate-laden gas stream. Generally, such approaches have used a slagging combustor concept for the removal of the bulk ash particulate. The gas turbine coal combustors operate at sufficiently high temperature by controlling the stoichiometry of the combustion air to near stoichiometric, in an adiabatic combustion chamber, so that ash becomes molten and is removed in the form of slag from the flue gas. This approach, however, retains significant amounts of residual fine ash particles (with an average size of 4 microns) in the gas stream which are sufficient to harm the turbine blades.
Slagging combustor systems also often utilize high temperature ceramic filters downstream of the gas turbine combustor and upstream of the turbine itself to capture residual fine ash particulates before they enter the turbine and erode or otherwise damage the turbine blades. Ceramic filters, however, admit a very low surface gas velocity, thus causing a large and unacceptable pressure drop across the filter. This causes the size of such ceramic filters to become prohibitively large and, therefore, very expensive. Furthermore, ceramic filters are unreliable because they are extremely fragile and susceptible to thermal shock and the thermal stresses resulting therefrom. In addition, such filters tend to plug, thereby requiring means for keeping the filters clean without causing a steady pressure drop across the filter as it "loads up" with fine particles.
The high temperatures at which the slagging combustors must operate also tend to increase the amount of nitrogen oxides produced in the combustion process. This, in turn, requires other means downstream from the coal combustor to reduce the concentration of nitrogen oxides in the effluent gas stream.
The high combustion temperatures in the slagging combustors operate at an inappropriate temperature for sulfur capture using dry sulfur sorbents such as limestone or dolomite. Sulfur oxides produced by burning the sulfur-laden coal fuels must, therefore, be removed from the flue gas stream somewhere downstream of the combustor. A further side product created by the high temperature in a slagging combustor is the release of alkali vapors in the gas stream that must also be removed to reduce corrosion of the turbine blades.
Other non-slagging designs utilize dry ash rejection upstream of the turbine. In these designs, sulfur is captured using dry sorbents in a tri-stage combustor. A multi-stage modular design of a combustion apparatus in this approach utilizes a modified tri-stage combustor modified for ash rejection and sulfur capture. An aerodynamic particle separator separates ash rejection. This system has been found to produce hard black deposits on the surface of the combustor quench zone. Involuntary slagging in the quench zone thus results, with hardened pieces breaking off and traveling downstream without resettling on other surfaces of the combustor which could damage the system. Further gas clean-up and nitrous oxides controls must also be employed downstream of the combustor.
Other systems utilize fabric-filter technology to control emissions in standard boiler applications. Fabric filters, however, are not applicable to the gas turbine hot gas clean-up systems.
In summary, effective reduction of suspended particulates in a gas stream created by combustion remains a paramount problem due to the lack of a cost effective, efficient system for particulate removal, particularly very small particulates. Available particulate collection/removal systems are limited by generator operating conditions. New innovative approaches are thus needed to provide a system so that fuel which produces particulates may be employed to operate generators requiring highly cleaned gases. Any such new system should possess a number of attributes, such as high combustion efficiency, high sulfur capture capability, high solid fuel particulate removal, low nitrogen oxide emissions, and high removal of alkali vapors created by the combustion of the fuel. Moreover, a new system providing the above attributes should also be relatively inexpensive and should not require substantial preparation and pre-cleaning of the fuel used for combustion.
Acoustic agglomeration is a process in which high intensity sound is used to agglomerate submicron- and micron-sized particles in aerosols. Agglomeration is, in essence, a pretreatment process to increase the size distribution of entrained or suspended particulates to enable high collection/removal efficiencies using cyclone or other conventional separators. Acoustic waves cause enhanced relative motion between the solid particles, and hence, increases collision frequency. Once the particles collide, they are likely to stick together. As an overall result of acoustic influence, the particle size distribution in the aerosol shifts significantly from small to larger sizes relatively quickly. Larger particles may be more effectively filtered from the carrying gas stream by conventional particulate removal devices such as cyclones. The combination of an acoustic agglomeration chamber with one or more cyclones in series provides a promising high-efficiency system to clean particulate-laden gases such as hot flue gases from pressurized combustors.
Acoustic agglomeration of small particles in hot combustion gases and other sources of fine dust-bearing effluent streams has been studied intermittently for many years. Although effective in producing larger-sized particles (5 to 20 microns) for more efficient removal by conventional devices, the prior art methods of acoustic agglomeration are not generally viewed as potential clean-up devices due to their large power requirements. For example, fine fly ash particulates (less than 5 microns in size) have been agglomerated using high-intensity acoustic fields at high frequencies in the 1,000-4,000 Hz range. These higher frequencies were necessary for the disentrainment of the fine particulate so as to effect collisions therebetween, and hence, agglomeration of the fine particles.
In such prior art acoustic agglomeration devices, the acoustic fields have been produced by sirens, air horns, electromagnetic speakers, and the like. The resulting acoustic wave generation for sonic agglomeration requires power estimated to be in the range of 0.5 to 2 hp/1,000 cfm. Significant parasitic power loss is therefore present as noted hereinafter even for efficient horns, sirens and the like which normally have efficiencies ranging from 8 to 10%.
Sirens, air horns and the like require auxiliary compressors to pressurize air needed to operate same. The pressure required is generally well above the pressure available at the gas turbine compressor exit, thus necessitating a means for providing that required pressure if the turbine is to be employed, or utilization of an auxiliary compressor. Electromagnetic sonic devices require special designs and precautions to provide the desired equipment reliability, availability and life. Likewise, powerful amplifiers are required to drive certain speakers in order to deliver 160 decibels (dB) or more of sound pressure. All of the above acoustic systems are thus inefficient from at least a cost standpoint.
Apparatus and processes according to the present invention overcome the above-noted problems of the prior art and possess the desired attributes set forth above by using a pulse combustor arrangement for acoustically enhanced bimodal agglomeration of particulate in a gas stream.